The present invention relates to an apparatus and a method for arranging magnetic solder balls one each on each of a plurality of bonding pads on a substrate through a mask.
Conventionally, solder paste is screen printed to form solder bumps on bonding pads for flip chip bonding of a substrate such as a wafer or a circuit substrate. However, in conjunction with the trend toward the higher density of semiconductor devices, the bump pitch is required to be infinitesimally small such as on the order of 200 μm or even 150 μm. If solder paste is screen printed at such an infinitesimally small pitch, attachment of the solder paste on a bridge between bumps and on a screen mask occurs. For this reason, the formation of the solder bumps at the infinitesimally small pitch cannot be effected by screen printing.
Accordingly, as a method of forming bumps at an infinitesimally small pitch, it is conceivable to arrange the solder balls on the bonding pads.
As one method which is conventionally carried out for bonding pads for board mounting on the outer side of a ball grid array (BGA), a method is known in which solder balls are sucked by a sucking jig and are transported so as to be mounted on the bonding pads. Adhesive soldering flux is coated in advance on the bonding pads, to thereby allow the solder balls to adhere to and fixed to the bonding pads. In this case, however, the diameter of the solder balls is large in the vicinities of 0.5 to 0.7 mm. In contrast, the diameter of the solder balls for the infinitesimally small pitch needs to be set to 100 μm or below. When the solder balls thus become infinitesimally small, there arise problems that the solder balls coagulate due to the action of static electric charge, and that the solder balls cannot be disposed at accurate positions owing to the effect of air flow. In addition, suction ports of the suction jig are also miniaturized, so that the processing cost becomes high, which is not suitable for practical use.
For this reason, a method in which the solder balls are dropped into openings of the mask placed on the substrate (a so-called “dropping method”) is carried out as another method. After the application of flux to the bonding pads on the substrate, the pads and the metal mask are positioned and superposed on top of each other. In that state, the solder balls are dropped from above the metal mask, and the solder balls are dropped into the openings of the metal mask. The solder balls are dropped from a container having an opening such as a slit, and it is ensured that an appropriate amount is dropped onto the mask. One ball is dropped into each opening of the mask, is placed on the bonding pad positioned with respect to the opening, and is adhered and fixed by adhesive flux applied to the bonding pad. The solder balls remaining on the mask are collected by a collecting mechanism.
Here, a means for reliably dropping the solder balls into the mask openings should preferably be provided. Patent document 1 discloses a method in which, by using a flexible spatula or brush as a dropping means, the solder balls are mechanically moved forcibly on the mask to drop the solder balls into the mask openings.
However, if the mechanical forcible movement by means of the spatula or the brush is carried out, there arise problems in that damage or deformation can occur in the solder balls owing to the spatula or the brush, and that foreign matter can be mixed in from the brush.
In recent years, cored solder balls such as Cu-cored solder balls and resin-cored solder balls have been used. The cored solder ball has a structure in which a solder layer is provided on the surface of a Cu or resin core, and a diffusion barrier layer is interposed between the core and the solder layer so that the solder in the surface layer is not diffused into the core under a high temperature during flip chip bonding. Here, the diffusion barrier layer is formed of a ferromagnetic material such as Ni.
This cored solder ball has outstanding advantages in that even if the solder is fused at the time of flip chip bonding, the Cu or resin core keeps its original shape and functions as a spacer for keeping a constant interval between an electronic part such as a semiconductor device and a mounting substrate and prevents faulty connection due to the cracking or deformation of the solder bump caused by high-temperature degradation.
However, if the mechanical forcible movement by means of the spatula or the brush is carried out, damage, deformation, and comingling of foreign matter due to it can occur, so that there has been a problem that full use cannot be made of the outstanding advantages of the cored solder balls.
[Patent Document 1] JP-A-2004-311886